1. Field of the Invention
The present invention relates generally to asphalt pavers and, more particularly, to asphalt pavers having laterally extendable screeds.
2. Description of the Related Art
Most paving machines for laying bituminous or asphaltic material roadways are of the so-called "floating screed" type. Each employs a tracked or wheeled tractor unit having a pair of rearwardly extending screed pull arms pivotally attached to the sides of the tractor unit. A screed assembly of the paving machine is attached to rearwardly projecting distal ends of the pull arms.
For this type of paving machine, the texture and density of a mat of asphaltic material placed by the paving machine is influenced by the weight of the screed assembly, which "floats" upon the asphaltic material therebeneath, and by the angular attitude of the underlying screeding surface of the screed assembly relative to the roadway, commonly referred to as the "attack angle" of the screed. For a given speed of the paving machine and the thicker the mat being laid by the machine, the greater the attack angle must be in order to achieve a desired density of the mat being placed by the machine. Hence the screed assembly, in turn, should be pivotally adjustable about a transverse horizontal axis such that the attack angle can be dynamically adjusted as operating conditions dictate.
A typical width of a main screed of the screed assembly of a paving machine for highway construction and the like approximates the overall width of the paving machine, or approximately ten feet for example. In order to lay a mat of greater width and thereby reduce the number of passes needed for a particular project, laterally extendable screed assemblies are commonly used.
Such extendable screed assemblies generally include a pair of shorter screeds, or "screed extensions", carried by and disposed rearwardly of the main screed. The screed extensions are generally attached to the main screed such that one or both of the screed extensions can be slidably adjusted outwardly from the main screed, thereby extending the effective width of the screed assembly. For example, the screed extensions may be extended outwardly such that the overall width of the screed assembly--the main screed plus two screed extensions, one to the left and one to the right--ranges up to approximately twice the width of the main screed, or approximately twenty feet for example. The overall width of the mat laid in a single pass of the paving machine is thereby increased. As a result and in terms of time and therefore cost of paving a given roadway, the efficiency of the paving machine is also increased by utilizing the extendable screeds.
Prior art paving machines utilizing screed extensions typically use an opposing pair of augers situated at the rear of the paver and extending perpendicularly outwardly to laterally distribute asphaltic material from immediately behind the paving machine to approximately the paths to be traversed by the outer extremities of the auger extensions. The outer ends of these augers generally are either unsupported or are supported off of the paving machine. The augers are generally pitched and rotated such that the asphaltic material is urged outwardly and forwardly relative to the direction of travel of the paving machine.
Various interrelated factors contribute to distribution of asphaltic material to the outer extremities of the auger extensions. Such factors include the rate at which asphaltic material is deposited by the paving machine on the subgrade at the rear of the paving machine, the speed of the paving machine, the extent to which the perpendicularly oriented auger extensions reach outwardly from the paving machine, the screw pitch of the augers, the rate of rotation of the augers, the thickness of the mat being laid by the paving machine, etc.
Due to the forward motion of the paving machine, the farther the auger extensions extend outwardly from the paving machine, the greater the amount of asphaltic material that must be accumulated and pushed in front of the augers in order for sufficient asphaltic material to be urged laterally in order to reach the outer extremities of the auger extensions in sufficient quantity to provide a mat of asphaltic material having the desired thickness at those outer extremities of the screed extensions. Unfortunately, the greater the quantity of asphaltic material that must be accumulated and pushed in front of the augers as aforesaid, the greater the power demands placed on the prime mover of the paving machine. Further, the greater the power demands placed on the paving machine for conveying the asphaltic material to the outer reaches of the screed extensions, the less the power available for desired traction requirements and for propulsion of the paving machine.
What is needed is an asphalt paving machine having means to laterally distribute asphalt material to outer extremities of extendable screeds thereof without unnecessarily loading a prime mover of the paving machine which, otherwise, would reduce the power available for traction requirements and propulsion of the paving machine.